1. Field of the Invention
The present invention relates to security for communication systems and more specifically to transmission level security for communication systems using multiple carrier modulation techniques. Even more specifically, the present invention relates to transmission level security for communication systems using multiple carrier modulation techniques, such as orthogonal frequency division multiplexing (OFDM).
2. Discussion of the Related Art
Security is becoming increasingly important in communications systems which transmit data. Wireline communication systems, such as coaxial cable and especially fiber optic cable, are inherently secure since such mediums must be physically tapped to intercept data transmissions. On the other hand, wireless data transmission is inherently insecure, since data is transmitted openly over an air interface such that an unauthorized entity may easily intercept the transmission. In the context of wireless local area network (LAN) applications, security is extremely important since sensitive data is transmitted to and from multiple devices that constitute the network. Without any security measures, eavesdroppers may freely receive data transmissions between devices in the wireless LAN. As such, transmission security is required to ensure that unauthorized entities are not allowed to intercept the data as it is communicated across the network.
Many bit-level security technologies exist in which the data is encrypted prior to transmission over the wireless medium. Typically, encryption involves scrambling the data bits prior to transmission and the receivers in the network are provided with the logic to decrypt the transmitted information. Common bit-level security techniques include DPF (Data Private Facility) and DES (Data Encryption Standard). These encryption techniques make use of either a Public Key or a Private Key which is used by authorized receivers to decrypt the data. As such, without the key, unauthorized users have a difficult time descrambling the data.
Many bit-level security measures, for example, those used in OFDM (Orthogonal Frequency Division Multiplexed) wireless LAN applications using the HiperLAN2 and IEEE 802.11 standards, offer data payload level encryption to encrypt the data but transmit the preambles and control fields with no protection. Thus, an unauthorized eavesdropper can easily intercept the data, although it is encrypted. Disadvantageously, having received the data, the unauthorized eavesdropper can easily parse the data fields from the PHY-layer communications and possibly reverse engineer the encryption technique and extract the underlying source information.
Another form of security is transmission-level security in which the physical waveform (i.e. PHY-layer) carrying the data is altered according to an encryption technique. Advantageously, since the physical waveform is altered, unauthorized eavesdroppers may not even be able to meaningfully receive the transmitted signal at all. One conventional transmission-level encryption technique includes frequency hopping in which the carrier frequency that carries the data is periodically hopped between several different frequencies in a pseudo random fashion. Unless an eavesdropper is aware of the frequencies used and the hopping pattern, the eavesdropper will be unable to receive the transmitted data.
The present invention advantageously addresses the needs above as well as other needs by providing a transmission level security technique which introduces a known group delay distortion into the transmitted signaling, which for the unequipped receiver, will interfere with the reception of data at the receiver.
In one embodiment, the invention can be characterized as a method of transmission security in a communication system and a means for accomplishing the method, the method comprising the steps: forming a plurality of digital signals representing a symbol to be transmitted over a communication medium, wherein respective ones of the plurality of digital signals are modulated onto respective ones of a plurality of subcarriers according to a multiple carrier modulation scheme; and introducing a predetermined group delay distortion in one or more of the plurality of subcarriers, such that portions of the one or more of the plurality of subcarriers will be received outside of a time window corresponding to the symbol at a receiver.
In another embodiment, the invention may be characterized as a method of transmission level security in a communication system comprising the steps: forming a plurality of digital signals representing a symbol to be transmitted over a communication medium, wherein respective ones of the plurality of digital signals are modulated onto respective ones of a plurality of subcarriers according to a multiple carrier modulation scheme; and introducing a group delay distortion in one or more of the plurality of subcarriers but less all of the plurality of subcarriers, such that portions of the one or more of the plurality of subcarriers will be received outside of a time window corresponding to the symbol at a receiver.
In yet another embodiment, the invention may be characterized as a method of transmission level security in a communication system comprising the steps: forming a signal to be transmitted over a communication medium; and introducing a predetermined group delay distortion in the signal, such that portions of the signal will be received outside of a time window corresponding to the signal at a receiver.
In another embodiment, the invention may be characterized as a method of transmission level security for time windowed communication systems comprising the steps: receiving a signal, wherein the signal has undergone a predetermined group delay distortion, wherein the predetermined group delay distortion is such that portions of the signal will be received outside of a nominal window of time corresponding to the signal at a receiver; and substantially removing the predetermined group delay distortion from the signal such that the signal fits within the nominal window of time.
In a further embodiment, the invention may be characterized as a transmitter implementing transmission level security for wireless communications comprising: a baseband modulator producing a signal to be transmitted to a receiver; and an encryptor coupled to the baseband modulator for introducing a predetermined group delay distortion in the signal such that portions of the signal will be received outside of a nominal window of time corresponding to the signal at the receiver.
In another further embodiment, the invention may be characterized as a receiver implementing transmission level security for wireless communications comprising: an antenna for receiving a signal, wherein the signal has been undergone a predetermined group delay distortion such that portions of the signal will be received outside of a nominal window of time corresponding to the signal at the receiver; a downconverter coupled to the antenna; and a decryptor coupled to the downconverter for substantially removing the predetermined group delay distortion from the signal such that the signal fits within the nominal window of time.
In another embodiment, the invention may be characterized as a method of transmission level security in a communication system comprising the steps: forming signals to be transmitted over a communication medium; introducing a predetermined group delay distortion in each of the signals, such that portions of the signals will be received outside of a nominal time window corresponding to the signals at a receiver, the predetermined group delay distortion selected from a plurality of predetermined group delay distortions; and changing the predetermined group delay distortion introduced in at least one of the signals to another one of the plurality of predetermined group delay distortions.
In yet another embodiment, the invention may be characterized as a method of transmission level security in a communication system comprising the steps: forming signals to be transmitted over a communication medium; and introducing a predetermined group delay distortion in each of the signals, such that portions of the signals will be received outside of a nominal time window corresponding to the signals at a receiver, the predetermined group delay distortion selected from a plurality of predetermined group delay distortions; wherein the predetermined group delay distortion introduced is hopped between respective ones of the plurality of predetermined group delay distortions.